SDCAR2010【逻辑入门】（八）Assumptions

sdcar2010

For the difference between sufficient assumption and necessargy assumption, here is  the analogy:

Suppose you have city A and city B, separated by a river H.  If there are multiple bridges over the river H, each bridge is sufficient but not necessary to connect city A with city B. What is necessary is that at least one bridge over river H is working.

If there is only one bridge over the river H, then that bridge is both sufficient and necessary to connect city A with city B.

If there there are two rivers C and D separate the cities A and B, and there is one bridge over C and many bridges over D, then the bridge over C is necessary (but not sufficient) to connect city A and B, while all the individual bridge over D is neither necessary nor sufficient to connect city A with city B. What is necessary is that at least one bridge over D is working!

曳烛

For the difference between sufficient assumption and necessargy assumption, here is  the analogy:

Suppose you can city A and city B, separated by a river H.  If there are multiple bridges over the river H, each bridge is sufficient but not necessary to connect city A with city B. What is necessary is that at least one bridge over river H is working.

If there is only one bridge over the river H, then that bridge is both sufficient and necessary to connect city A with city B.

If there there are two rivers C and D separate the cities A and B, and there is one bridge over C and many bridges over D, then the bridge over C is necessary (but not sufficient) to connect city A and B, while all the individual bridge over D is neither necessary nor sufficient to connect city A with city B. What is necessary is that at least one bridge over D is working!

-- by 会员 sdcar2010 (2012/3/18 23:13:05)

呃，好吧，真的是相当形象的比喻。。我想我应该明白了。。。嗯，不过还是再说下我的理解SDCAR你看下有没有问题哈。


就是说过C河的那个桥是C选项，过D河的那些个桥之一是B选项，过D河的另一座桥是“除了穷人和富人外其他类别的人也是以相同的相对比例没有被统计到”。因此即使我们否定了B选项（这座桥断了），但这并不阻碍我们从A城市到达B城市（假设B为结论）。。。。。好吧，如果我的理解没有错误的话我想我真的是学到不少东西了。。  = =！


另外还有一点想要问一下啊，就是前面你说过“The percentage of poor Americans uncounted by the census is close to the percentage of rich Americans uncounted.”表达的是相对的百分比，那么如果我要表达绝对百分比，则应该怎样表述呢（我总觉的这个是个绝对的表述方式           = =！）

jxgzcly

楼主,换个美女头像.... 我原动力下降一大半. 看的这闪来闪去的,我要晕死了~~~ 求同情,求养眼.

PhoebeQi

For question No.2
2. A researcher discovered that people who have low levels of immune-system activity tend to score much lower on tests of mental health than do people with normal or high immune-system activity. The researcher concluded from this experiment that the immune system protects against mental illness as well as against physical disease.
The researcher's conclusion depends on which of the following assumptions?
(A) High immune-system activity protects against mental illness better than normal immune system activity does.
(B) Mental illness is similar to physical disease in its effects on body systems.
(C) People with high immune-system activity cannot develop mental illness.
(D) Mental illness does not cause people's immune system activity to decrease.
(E) Psychological treatment of mental illness is not as effective as is medical treatment.

But if you negate C. people with high immune-system activity CAN develop mental illness. If that is true, then the conclusion that the immune system protects against mental illness is wrong. So why choose D but not c?

-- by 会员 glendasc (2011/7/15 23:25:01)

If you negate C), the conclusion still holds since the conclusion is "the immune system PROTECTs against mental illness,", which means people with high immune system have some protection but that protection is not bullet-proof. So people with high immune system could become mentally unstable, but less likely than those who have low immune system.

-- by 会员 sdcar2010 (2011/7/16 0:02:26)

So what if we negateA)High immune-system activity Does NOT protect against mental illness better than normal immune system activity does.
if this is the case, "the immune system protects against mental illness as well as against physical disease" must be wrong, right? we can conclude immune system does not protect against MI

PhoebeQi

oh I got it!!

echosangsang

dear scdar, i can't understand   (1)   "This is in contrary to the conclusion of the stimulus -- Drivers who equip their vehicles with radar detectors ARE more likely to exceed the speed limit regularly than are driver who do not. So if you negate (B), the stimulus falls apart. Thus, (B) is the correct answer."  please help me !!!

sdcar2010

1. A recent report determined that although only 3 percent of drivers on Maryland highways equipped their vehicles with radar detectors, 33 percent of all vehicles ticketed for exceeding the speed limit were equipped with them. Clearly, drivers who equip their vehicles with radar detectors are more likely to exceed the speed limit regularly than are drivers who do not (conclusion).
The conclusion drawn above depends on which of the following assumptions?
(A) Drivers who equip their vehicles with radar detectors are less likely to be ticketed for exceeding the speed limit than are drivers who do not.
(B) Drivers who are ticketed for exceeding the speed limit are more likely to exceed the speed limit regularly than are drivers who are not ticketed.
(C) The number of vehicles that were ticketed for exceeding the speed limit was greater than the number of vehicles that were equipped with radar detectors.
(D) Many of the vehicles that were ticketed for exceeding the speed limit were ticketed more than once in the time period covered by the report.
(E) Drivers on Maryland highways exceeded the speed limit more often than did drivers on other state highways not covered in the report.

If you negate (B), you have a statement A:
Drivers who are ticketed for exceeding the speed limit are NOT more likely to exceed the speed limit regularly than are drivers who are not ticketed.

If statement A is true, then for those drivers who have radar detectors and who got a speeding ticket, they are NOT more likely to exceed the speed limit regularly than are drivers who are not ticketed. This is in contrary to the conclusion of the passage -- Drivers who equip their vehicles with radar detectors ARE more likely to exceed the speed limit regularly than are driver who do not. So if you negate (B), the stimulus falls apart. Thus, (B) is the correct answer since B) is necessary for the conclusion to hold.

echosangsang

An assumption is an unstated premise. In other words, an assumption is a piece of evidence that strengthens the author’s argument but that the author never sates explicitly in the stimulus.

Sufficient vs. Necessary Assumptions

Imagine that you were accused of cheating during a GMAT test. At the hearing before a panel of disciplinary committee, only one piece of evidence is presented – your one-sentence testimony: “I did not cheat at the GMAT exam.” After hearing your testimony, the panel concludes that you did not cheat. You are definitely elated. But logically, the panel’s conclusion is on shaky ground because they assume, for better or worse, that you are telling the truth! In fact there are at least three different assumptions the panel could be making.
1. You are telling the truth.
2. You told the truth at the hearing.
3. It is possible for you to tell the truth.

The first assumption that you always tell the truth is a sufficient assumption because it is sufficient or enough to justify the panel’s conclusion. If you always tell the truth, then the panel must be right. However, this assumption is not a necessary assumption or an assumption that we have to make, because the panel could still be right even if you do not always tell the truth. As long as you told the truth at your hearing, the judge is still right. In other words, the panel’s conclusion does not depend on the assumption that you always tell the truth.

The second assumption that you told the truth at your hearing is both a necessary and sufficient assumption. It is sufficient because if you told the truth at the hearing, then the panel is right; it is also necessary because if you did not tell the truth at the hearing, then the panel is wrong. In other words, the panel’s decision depends on this assumption being true.

The third assumption that it is possible for you to tell the truth is a necessary assumption because if it were not possible for you to tell the truth, then the panel would be wrong. However, this assumption is not sufficient because it alone does not justify the panel’s conclusion. Just because you can tell the truth does not mean you actually tell the truth at the hearing. Therefore, the panel could still be wrong, even if the last assumption is true.

As this example illustrates, sufficient assumptions typically have strong language (always, never, none, all), while necessary assumptions typically have weak language (possible, likely, often, many).

Sufficient Assumption

Prompts for sufficient assumptions
? Which one of the following, if assumed, would justify the conclusion?
? The author’s conclusion would be properly drawn if which one of the following was true?

Sufficient assumption prompts almost always use the word IF, while necessary assumption prompts never use IF. Also, sufficient assumption prompts avoid qualifiers such as MOST. MOST JUSTIFIES, for example, is a strengthen question, while JUSTIFY by itself is a sufficient assumption question.

Before you look at the answers:
1. Pinpoint the main conclusion in the passage. (Read my previous Main Point post.)
2. Separate the premises from everything else. After you find the main point, don’t assume that all the other statements are premises; they might include opposing viewpoints, background information, and concessions.
3. Are there any jumps between the premises and the conclusion or between each premise? To find these jumps, look for any concepts that are discussed only once. There are usually two. The correct answer usually fills in the gap in the argument by linking those two concepts together. Alternatively, slowly read each word of the conclusion. Which phrase was not used in the premise? That word or phrase will likely reveal where the argument jumped to the conclusion: at the gap!

Look for the answer that links those two concepts together.
1. The correct answer will almost always use the exact same language used in the passage to describe each concept. If “many smart CDers” were one the concepts you found in the gap, the correct answer would likely use those three exact words, while the wrong answers would likely use similar but different words such as “many diligent CDers” or “smart CDers who read SCAR’s posts.” For the former, diligent is different from smart; for the latter, it refers to a smaller subset of CDers than the entire groupof “smart CDers.”
2. Because a sufficient assumption must justify the conclusion, the correct answer often has all, only, no, none, most, always, or some other strong words.

Jumps in Reasoning

Jumps can occur between the premises and the conclusion or between each premise. Here is a jump between premises:
1. Our school requires some of the highest tuitions in the country.
2. High tuitions in any school discourage poor students from attending.
3. Discriminatory schools cannot attract the best and the brightest students.
Therefore, our school cannot attract the best and the brightest students.

The first and second premises are related, but not perfectly: schools that impose the highest tuitions do not necessarily impose high tuitions – only tuitions that are higher than other schools. That said, it is not unreasonable to assume that a school with some of the highest tuitions in the country would have high tuitions. So this is a jump, but it is a small one. The second and third premises makes a larger jump: a school discourages poor students from attending is not necessarily a discriminatory school.

So in a sufficient assumption question, the correct answer for this argument would likely link these last two phrases together by saying that “any school that discourages poor students from attending is a discriminatory school” or “only discriminatory schools discourage poor students from attending.” Note that both expressions say the same thing because any means if and only means then.

Necessary Assumption
Prompts for sufficient assumptions:
? The conclusion relies on which one of the following assumptionps?
? The argument presupposes which one of the following?
? The conclusion does not follow unless

Before you look at the answers:

1. Pinpoint the main conclusion in the passage. (Read my previous Main Point post.)
2. Separate the premises from everything else. After you find the main point, don’t assume that all the other statements are premises; they might include opposing viewpoints, background information, and concessions.
3. Are there any jumps between the premises and the conclusion or between each premise? To find these jumps, look for any concepts that are discussed only once. There often two such concepts in the same passage. Unlike the jumps in sufficient assumption passages, however, the jumps in necessary assumption passages are often less obvious.

Look for the answer that must be true for the conclusion to stand.

1. Ask yourself, “Does this answer have to be true?” Or could the conclusion still be true without it? In other words, if the answer were not true, would it undermine the conclusion? Is it an assumption that the conclusion depends on?
2. Negate your last two best answers choices. The negated answer that undermines the conclusion is the correct answer.
3. If you spot any jumps, the correct answer will often link those two concepts together.

Negating an answer choice

Imagine you are eating ice cream anyour friend asks you how it tastes. You reply, “It is sweet.” The negation of this statement is that the ice cream “is not sweet.” The complete opposite of this statement is that the ice cream “is sour.”

When you negate an answer, just negate it. Try not to turn it into its opposite. Also, negate either the verb or the quantity, but not both.

SDCAR2010【逻辑入门】（七) Strengthen and Paradox

SDCAR2010【逻辑入门】（九）Flaw (part 1)

-- by 会员 sdcar2010 (2011/6/24 2:47:27)

亲爱的SDCAR，你知道GMAT中还有一种题目是evaluate，你能说下这种题型应该怎么办吗？

eejqchen

非常感谢，可以拿去出本教材了

twwzx

3. In Kravonia, the average salary for jobs requiring a college degree has always been higher than the average salary for jobs that do not require a degree. Current enrollments in Kravonia's colleges indicate that over the next four years the percentage of the Kravonian workforce with college degrees will increase dramatically.  Therefore, the average salary for all workers in Kravonia is likely to increase over the next four years.
Which of the following is an assumption on which the argument depends?
A. Kravonians with more than one college degree earn more, on average, than do Kravonians with only one college degree.
B. The percentage of Kravonians who attend college in order to earn higher salaries is higher now than it was several years ago.
C. The higher average salary for jobs requiring a college degree is not due largely to a scarcity among the Kravonian workforce of people with a college degree.
D. The average salary in Kravonia for jobs that do not require a college degree will not increase over the next four years.
E. Few members of the Kravonian workforce earned their degrees in other countries.


i wonder "Therefore, the average salary for all workers in Kravonia is likely to increase over the next four years." should be the conclusion or the inference. thks